A typical image sensor includes a pixel array with multiple pixels, each having a photosensitive area, and supporting circuitry. Conductive lines that carry electrical signals among and between pixels are typically formed in dielectric layers over the front side of the pixel. In front-side illuminated (FSI) pixels, the conductive lines can reduce the amount of incident light reaching each pixel. Backside-illuminated (BSI) image sensors are image sensors where the light from the image to be captured is incident on the backside of the image sensor's pixel array instead of the front. One of the advantages of BSI image sensors is that the incident light can reach each pixel's photosensitive area without having to traverse the space between conductive lines on the front side of the pixel array.
One phenomenon that has been observed in BSI image sensors is that light reaching the pixels through the backside can be reflected by the interfaces between the various layers of substrate and dielectric that make up the image sensor, as well as being reflected by the conductive lines themselves. Among other things, this phenomenon can cause unnecessary and unwanted “ghosting” in images captured by the image sensor. The phenomenon can occur at any wavelength, but is more pronounced for longer wavelengths such as wavelengths in the red part of the spectrum.